1. Field of the Invention
The present invention relates to a signal generator for obtaining a frequency signal.
2. Description of the Related Art
A device for generating an arbitrary frequency signal is required, for example, in order to obtain a clock signal according to a system, and a method of combining a D/A (digital/analog conversion unit) and DDS (Direct Digital Synthesizers) is known in order to obtain an analog frequency signal based on a frequency setting signal composed of a digital signal. When receiving an inputted digital signal corresponding to a set frequency, the DDS in which data for creating a sinusoidal signal is written in a ROM table, reads an address in a read pattern according to the input value and thereby outputs a sinusoidal signal corresponding to the set frequency from the ROM table. The output is converted into an analog signal in the D/A conversion unit and thereby the objective analog frequency signal is obtained.
In this case, it is necessary to increase the resolution for data in the ROM table in order to improve the noise characteristics such as spurious and the like of the generated signal, which requires a large memory capacity and a large consumption current.
Patent Document 1 discloses a technique of generating a rectangular wave of an objective frequency based on a triangular wave of digital data. In this technique, after D/A conversion of the digital data, the analog triangular wave is linearly interpolated. The purpose of linear interpolation is to fix the zero cross timing to a timing according to the frequency because the timing of the analog triangular wave crossing a zero point cannot be made more accurate than a sample timing of the digital signal (paragraphs 0019 to 0023). Then, a comparator is used to obtain the zero cross timing of the analog triangular wave to obtain an output (paragraph 0024).
Patent Document 1 states that a digital signal is inputted into an adder to generate a sawtooth wave proportional to input and time, then the sawtooth wave is converted into a triangular wave in a triangular wave conversion circuit, the triangular wave is D/A converted, the analog triangular wave is linearly interpolated, and the output of a linear interpolation circuit is further binarized by a predetermined voltage in a comparator to obtain an arbitrary frequency signal.
However, an operational amplifier is necessary for a sample hold circuit used for linear interpolation, and there is a problem that when the operational amplifier being an active element is used, the fluctuation in gain causes a waveform distortion.    Patent Document 1: Japanese Patent Application Laid-open No. Hei 5-206732 (paragraphs 0019 to 0024, FIG. 1)